Interferons have produced objective regressions in several human cancers. The basis for this antitumor action is unknown and is of considerable therapeutic and scientific importance since the interferons are naturally occurring cell regulatory substances. Treatment of cells with homologous interferons results in the induction of 2', 5' oligoadenylate synthetase. Upon activation by double-stranded RNA, this enzyme synthesizes a series of 2', 5' linked oligonucleotides which, in turn, activate a latent cellular endoribonuclease and profoundly inhibit protein synthesis. However, the role of the 2-5A system in the response of cells to interferon is unclear, and this research will study one aspect of the problem: the actions of the 2-5A dependent endoribonuclease, in vitro and in human tumor cells. Specifically, we will examine the inhibition of cell-free protein synthesis by 2-5A to determine whether this effect occurs primarily through cleavage of messenger RNA or ribosomal RNA, both of which are known targets for the enzyme. The functional significance of the 2-5A mediated cleavage of ribosomal RNA will be determined by examining the 2-5A sensitivity of the cell-free translation of single-stranded DNA (which occurs in the presence of an aminoglycoside antibiotic). The optimum treatment conditions for 2-5A mediated rRNA cleavage will be determined for human tumor cells grown in culture. Finally, using cDNA probes directed at human and murine ribosomal RNA, we will examine ribosomal RNA cleavage in human tumor xenografts in athymic mice treated with various interferons and attempt to correlate this effect with tumor regression. The significance of this work to the treatment of human cancer will be a better understanding of the molecular mechanisms of action of the interferons, and possibly a contribution to the design of rational interferon therapies.